Dear D.,
Just like in English, I can't think of ordinary situations in which "His parents kissed themselves" would be used, but let's imagine they're under a spell, and start kissing themselves frantically ;)
The fact is that without a clear context, the French sentence "Ses parents se sont embrassés" would be the way to express this idea, as well as it could also mean "His parents kissed each other", we agree a more common occurrence :)
I hope that's helpful!
À bientôt !